Massive black holes use jets to stomp out stars

Finding solves long-standing problem in galaxy-formation models

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In typical massive post-starburst galaxies, the recent star formation has abruptly halted. Astronomers think the recent star formation was driven by interactions with other galaxies, and so the galaxies would show disturbed appearances (shown here).

ST. LOUIS — The universe's hefty black holes are known to devour everything within their reach. Now astrophysicists have found that some of the most massive of these sinkholes use high-energy jets to stomp out nearby star formation.

The finding, presented today here at a meeting of the American Astronomical Society (AAS), solves a long-standing problem in galaxy-formation models.

"The problem is that when you run these models and compare them to observations, what you find is that the models over-predict stellar mass in low-mass galaxies and in high-mass galaxies," said researcher Sugata Kaviraj, an astrophysicist at Oxford University in England. "There are too many stars."

In less massive galaxies, exploding stars were added as the party crashers, imparting enough energy to disperse close-knit bundles of gas that fuel star formation.

When a galaxy beefs up to the equivalent of 10 billion suns, however, supernovae don't have enough power to throw out star-forming gas. As the mass of a galaxy increases, the gravitational pull holding onto that gas also soars, making it tougher to give that gas the boot. And so theorists figured that in the beefier galaxies, supermassive black holes, thought to reside at the centers of most galaxies, could take over and stomp out star formation.

As gas falls into the gravitational clutches of a supermassive black hole, the energy gets spit out as a pair of laser-like jets from either end of the black hole. These active black holes, called active galactic nuclei (AGN), have enough energy to power 10 billion stars like the sun. They also can kick out gas from hefty galaxies, theory had suggested.

"The problem was that people used these supernovae and AGN prescriptions to match the models to the observations," Kaviraj told SPACE.com. "However, there was no evidence that this is actually how nature works."

To nail down that evidence, Kaviraj and his colleagues looked at the level of star-snuffing taking place in so-called post-starburst galaxies, which show evidence of recent (within 1 billion years) star formation that was abruptly halted. They used ultraviolet data collected by the orbiting space telescope, NASA's Galaxy Evolution Explorer, and optical images from the Sloan Digital Sky Survey.

The team studied the relationship between galaxy mass and rate of star quenching, which in simple terms means the number of stars snuffed out in a certain period of time. If supernovae were the only means of kicking out gas, Kaviraj would expect the rate of star quenching to decrease as the galaxy mass goes up.

And that's what the researchers found for galaxies below 10 billion solar masses. Above this weight, they found the opposite: As mass increased, star quenching also boosted dramatically.

Kaviraj said these observations support the AGN/jets explanation in the most massive galaxies. Once galaxies grow to about 10 billion solar masses, when supernovae can't kick out gas, active supermassive black holes take over. The black hole's powerful jets must be kicking out the gas, at least in the post-starburst galaxies studied, he said.

Astronomers also have found that galaxies weighing more than 10 billion suns are more likely to have AGNs than galaxies lighter than 10 billion suns.

While the current results are based on nearby galaxies, the researchers hope to widen the scope of the work to include more distant galaxies that date back to the peak epoch of star formation some 10 billion years ago, when the universe was only 25 percent of its current age.

This research was funded by a Leverhulme Early-Career Fellowship, a BIPAC fellowship at the University of Oxford and a Research Fellowship at Worcester College, Oxford.